I have read elsewhere that these plates were coloured by pochoir/stencil,
i.e. by hand with stencils. There are certainly a few smudges here and there which might suggest this.
Grasset was used to working with cutting-edge colour printers like the engraver Charles Gillot, pioneer of colour artwork for books like Grasset's Quatre Fils Aymon.
Is this really stencilled/hand-coloured?
I'd love to know.